1. Field of the Invention
The present invention relates to a color cathode-ray tube, and more particularly to a color cathode-ray tube with an enhanced visibility, wherein the mechanical strength of the mask body is improved.
2. Description of the Related Art
In general, in a color cathode-ray tube, three electron beams that are emitted from an electron gun assembly are deflected in a horizontal direction and a vertical direction, while they are being converged. Thereby, a color image is displayed. A deflection coil that generates deflection magnetic fields for deflecting the electron beams comprises at least a pair of horizontal coils and a pair of vertical coils. The convergence characteristics of the three electron beams are substantially determined by the deflection magnetic fields that are generated by the deflection yoke. Thus, as is generally known, the horizontal deflection magnetic field is formed of a pincushion type magnetic field and the vertical deflection magnetic field is formed of a barrel type magnetic field.
In actual cases, in order to correct coma aberration, etc., the horizontal deflection field has a barrel shape on an electron gun-side part thereof and a pincushion shape on a phosphor screen-side part thereof, and thus the horizontal deflection field is formed in a pincushion shape as a whole. In addition, the vertical deflection field has a pincushion shape on an electron gun-side part thereof and a barrel shape on a phosphor screen-side part thereof, and thus the vertical deflection field is formed in a barrel shape as a whole.
In this case, since the vertical deflection field has a barrel shape as a whole, pincushion-type image distortion occurs in the vicinity of a horizontal-axis end part on the screen, that is, in the vicinity of a minor-axis side part. In a case where the degree of pincushion-type distortion is great due to an influence of flattening of the screen, etc., a deflection current waveform is corrected in usual cases, thereby correcting the pincushion-type distortion.
In this color cathode-ray tube, in order to display a color image, which is free from color misregistration, on the phosphor screen, it is necessary that the three electron beams, which have passed through electron beam passage holes formed in a mask body of the shadow mask, correctly land on the associated three-color phosphor layers on the phosphor screen. To achieve this, the shadow mask needs to be exactly disposed at a predetermined position relative to the panel. In short, a distance (q-value) between the panel and shadow mask needs to be exactly and properly set.
In recent years, in order to enhance the visibility of color cathode-ray tubes, there is a demand for a decrease in curvature (i.e. an increase in radius of curvature) of the outer surface of the panel to a level of a flat plane. Accordingly, it becomes necessary to similarly decrease the curvature of the inner surface of the panel from the standpoint of visibility. Further, in order to cause the electron beams to exactly land on the phosphor layers on the inner surface of the panel, it is necessary to properly set the q-value, as mentioned above. Moreover, the curvature of the mask body having electron beam passage holes needs to be decreased in accordance with the inner surface of the panel (see, e.g. Jpn. Pat. Application. KOKAI Publications Nos. 11-242940 and 11-288676).
If the curvature of the mask body is set at a small value, however, the mechanical strength of the shadow mask itself would decrease. Consequently, in the fabrication process of the cathode-ray tube, deformation of the shadow mask, for instance, would occur. Such deformation of the mask body leads to displacement of beam landing. If the electron beam shifts beyond a black non-emission layer due to the displacement of beam landing and the beam causes a phosphor layer, which is other than the phosphor layer of the color associated with this beam, to emit light, considerable degradation would occur in color purity.
In the case where the curvature of the inner surface of the panel is set at a large value in accordance with the curvature of the mask body, the fabrication of the panel becomes difficult and degradation in visibility, such as a decrease in luminance at the peripheral part of the panel, occurs. It is desirable, therefore, that the curvature of the inner surface of the panel be as small as possible.
With the flattening in panel shape, the aforementioned pincushion-type distortion near the horizontal-axis end of the screen increases. This problem cannot sufficiently be solved by the above-mentioned correction of the deflection current waveform. To solve this problem, there is a method wherein on the phosphor screen side of the deflection yoke, a magnetic member is employed to lead a leak magnetic field of the vertical deflection coil out toward the funnel side of the deflection yoke, and a pincushion-shaped magnetic field is produced in addition to a barrel-shaped vertical deflection magnetic field. Thereby, the pincushion-type distortion on the screen is corrected.
This method, however, cannot completely deal with the problem, and a permanent magnet for correction needs to be disposed on the phosphor screen side of the deflection yoke. In this case, if the mask body is designed to match with such deflection fields, the curved-surface strength of the mask body would considerably deteriorate.
As described above, if the curvature of the outer surface of the panel is decreased in order to enhance visibility and the pincushion-type distortion on the screen is corrected by the deflection yoke, the curvature of the inner surface of the panel is decreased and the curvature of the mask body is decreased. As a result, the mechanical strength of the mask body will decrease. Hence, the deformation of the mask body that occurs in the fabrication process and due to external shock leads to displacement in beam landing, and the color purity of the color cathode-ray tube considerably deteriorates.
In the case where the curvature of the inner surface of the panel, as well as the curvature of the mask body, is set at a large value in order to solve the above problem, the luminance at the peripheral part of the panel cannot be made uniform and the flatness may be lost. In some cases, it is difficult to completely correct-a distortion on the screen.